As wireless communication becomes increasingly common, more new frequency ranges are needed for different wireless systems. Meanwhile the demand for wireless terminal equipment, such as mobile stations, supporting several wireless systems is also on the increase. The most recent mobile station models typically employ several of the following systems and frequency ranges: EGSM 900 (880 to 960 MHz), GSM 1800 (1710 to 1880 MHz), GSM 1900 (1850 to 1990 MHz), WCDMA 2000 (1920 to 2170 MHz), US-GSM 850 (824 to 894 MHz), US-WCDMA 1900 (1850 to 1990 MHz) and US-WCDMA 1700/2100 (Tx 1710 to 1770 MHz, Rx 2110 to 2170 MHz). GSM 1900 and some WCDMA frequency ranges, for example, then at least partly overlap.
In small radio devices, such as mobile stations, the aim has often been to implement transmission and reception in all systems and frequency ranges by means of a single antenna. The small radio devices provide little space, so it would often be justifiable to use only one antenna. In such a case, however, different frequency ranges have to be combined to a common antenna by means of a lossy switch. The problem is particularly serious in connection with a WCDMA system wherein the use of a single antenna both for transmitting and receiving requires a duplex filter since transmission and reception take place simultaneously. In US-WCDMA 1900, for example, the duplex separation of the frequencies between transmission and reception is very small, so due to the strict filtering requirements, a duplex filter with as small losses as possible, such as a ceramic duplexer, has to be used. Such a duplex filter is considerably large and, in addition, it is typically advantageously installed underneath an antenna, which means that the antenna is provided with little space and the radiation efficiency of the antenna drops.
Therefore, both for the size of a mobile station and minimization of losses, it would be more advantageous to use an antenna structure comprising two antennas and to divide the transmission and reception e.g. in the WCDMA system between different antennas. This would enable the large, loss-incurring duplex filter to be avoided and replaced by simpler band-pass filters.
In such a solution, a problem is presented by the above-mentioned overlapping frequency ranges wherein simultaneous transmission and reception take place. The two antennas, or more precisely two radiators, provided in the single antenna structure and operating at least partly within the same frequency range couple strongly with each other during use. This means that when power is fed to a first radiator, some of this power transfers to a second radiator, which impairs the radiation power of both radiators and causes additional power consumption for the mobile station. In other words, isolation between the two antennas, i.e. radiators, is insufficient, typically of the order of less than 10 dB.
The applicant's earlier European Patent application 1 202 386 discloses a planar antenna structure for a radio device, wherein a planar radiator comprises at least one electrically non-conductive groove to enable the planar radiator to be divided into at least two parts, the frequency ranges provided by the two parts preferably being different. Such an antenna structure is advantageous e.g. in multifrequency mobile stations, but it cannot be used without losses for simultaneous transmission and reception taking place within the same frequency range; neither can the isolation problem described above be solved by such a structure alone.